Use correct representation to search on rays

The rays of grid vertices recently switched to storing vertex integer coordinates (on a grid) instead of vertex positions. While most of the code was adjusted, I overlooked one case when the rays are used to determine the material for sparse vertices that do not have any materials assigned to them. In this case, the material was borrowed from the nearest neighbour on a ray.

With the new change, old floating point positions were compared to the new vertex integer coordinates. This meant that for these vertices the labels were always assigned as outside as the coordinate check placed them always on the start of the ray.

This change removes this incorrect behaviour, addressing #16 (closed).